[docs] Reduce warnings

Use markdown syntax for inline code blocks instead of %true, %false, and
%NULL.

src/hb-draw.cc

diff --git a/src/hb-draw.cc b/src/hb-draw.cc
index fdc7110..74952b9 100644
--- a/src/hb-draw.cc
+++ b/src/hb-draw.cc
@@ -137,7 +137,7 @@
  * Return value: (transfer full):
  * A newly allocated #hb_draw_funcs_t with a reference count of 1. The initial
  * reference count should be released with hb_draw_funcs_destroy when you are
- * done using the #hb_draw_funcs_t. This function never returns %NULL. If
+ * done using the #hb_draw_funcs_t. This function never returns `NULL`. If
  * memory cannot be allocated, a special singleton #hb_draw_funcs_t object will
  * be returned.
  *
@@ -234,7 +234,7 @@
  *
  * Checks whether @dfuncs is immutable.
  *
- * Return value: %true if @dfuncs is immutable, %false otherwise
+ * Return value: `true` if @dfuncs is immutable, `false` otherwise
  *
  * Since: 4.0.0
  **/